Diabetes is a medical condition wherein the production and use of insulin is impaired, causing glucose levels to increase in the bloodstream. The most common types of diabetes are Type I (i.e. insulin-dependent diabetes) and Type II diabetes (i.e. non-insulin-dependent diabetes), also called diabetes mellitus. Impaired glucose tolerance (IGT) pertains to a medical condition wherein blood glucose levels that are higher than normal, but generally below the level of diabetes. More specifically, IGT is a combination of impaired secretion of insulin and reduced insulin sensitivity (i.e. insulin resistance). Some patients who develop IGT revert to normal glucose tolerance. Others remain in a chronic state of IGT. However, once IGT has developed, insulin secretion and sensitivity usually continue to decline, resulting in diabetes mellitus.
People with IGT are rarely treated because the condition is rarely diagnosed. Accordingly, it would be desirable to provide improved techniques for early detection of IGT so as to permit prompt treatment and it is to that end that the present invention is generally directed. Many patients susceptible to IGT, particularly the elderly, have pacemakers, ICDs or other implantable medical devices implanted therein, or are candidates for such devices. Briefly, a pacemaker is an implantable device that recognizes various arrhythmias such as an abnormally slow heart rate (i.e. bradycardia) or an abnormally fast heart rate (i.e. tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is an implantable device that additionally or alternatively recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate such fibrillation. Pacemakers and ICDs detect arrhythmias by sensing internal electrical cardiac signals using leads implanted within the heart. The internal signals comprise an intracardiac electrogram (IEGM). State-of-the-art pacemakers and ICDs are also typically capable of detecting a wide range of other parameters indicative of the physiology of the patient.
As many patients susceptible to IGT already have a pacemaker or ICD implanted therein, or are candidates for such devices, it would be desirable to provide techniques for detecting IGT using a pacemaker or ICD. Detection using a pacemaker or ICD would permit early detection of the condition. In turn, early detection permits early treatment, which improves the overall prognosis for the patient. In particular, early detection and treatment of IGT helps prevent the onset of diabetes mellitus through appropriate diet or therapy. Heretofore, however, the Applicant is unaware of any techniques for use with pacemakers, ICDs or other implantable medical devices that are specifically directed to detecting or evaluating IGT within the patient. Accordingly, certain aspects of the present invention are directed to filling that need. Other aspects of the invention are directed to techniques for detecting or evaluating diabetes mellitus within the patient, should that condition nevertheless arise. Still other aspects are directed to detecting the risk of mortality arising due to IGT or diabetes mellitus within the patient. Herein, IGT and diabetes mellitus are collectively referred to as “abnormal glucose level medical conditions.”